In subsea installations for use in, for example, controlling the extraction of oil or gas from subsea wells, it is usual for a surface located control station to communicate via an umbilical with a subsea located distribution unit which, in turn, communicates with a number of subsea located devices. The umbilical comprises a plurality of separate electrical power and communications lines. Depending upon the application in which the installation is used, the power lines may carry AC or DC supplies. It may also include hoses, fibre optic cables or the like. Power and communications signals may be transmitted between the surface location and the subsea located devices in this fashion. In some more complex architectures, additional distribution units may be provided between the primary distribution unit and the subsea located equipment.
In some arrangements, transformers may be present within the system, most typically within a distribution unit. Such transformers provide galvanic isolation and/or voltage transformation. However, transformers of this type are not always present.
The connections between the distribution unit or units and the subsea located equipment are typically made by the use of so-called jumpers. The jumpers typically comprise a plurality of electrical conductors for use in the transmission of power and communications signals. In addition, hoses or the like, or optical fibres, may also be present. Likewise, where additional distribution units are provided, similar jumpers or umbilicals may be used to provide the connections between the distribution units.
It is important for the electrical conductors of the umbilical, and the jumpers, in such arrangements to be electrically insulated from the sea water in which they are deployed. Failure of the insulation material can result in one or more of the conductors being shorted to ground, interfering with the operation or control of the subsea located equipment. The failure may arise from an impact, through gradual breakdown of the insulation or for other reasons.
In the event of such a failure, in order to identify the location of the fault it is usual for the entire installation to be powered down, and for a diver or ROV to be used to disconnect parts of the installation. Upon subsequent powering up of the remainder of the installation, if it is sensed that the short circuit has been alleviated then the operator will conclude that the fault lies within the part of the installation that has been disconnected. By repeated powering up and down, and disconnection and reconnection of parts of the installation, the location of the fault can be ascertained. Whilst this technique for identifying the location of a fault is useful, identification of the locations of faults in this manner is a time consuming and expensive operation. Furthermore, there is the risk that the disconnection and reconnection of parts of the installation may itself damage parts of the installation and so generate additional faults.
Whilst the description hereinbefore is concerned primarily with the detection of the location of faults arising from the failure of the insulation of an umbilical or jumper, the same technique can be used where the failure has occurred within a piece of subsea located equipment or within a distribution unit, or at a connection between an umbilical or jumper and a distribution unit or subsea device.